Environmental stress due to salinity is one of the most serious factors limiting the productivity of agricultural crops, which are predominantly sensitive to the presence of high concentrations of salts in soil. It is estimated that 35–45% of the 279 million hectares of land irrigation is presently affected by salinity. This is exclusive of the regions classified as arid and desert lands. In this century, more areas including vast regions of Australia, Europe, Southwest USA, the Canadian prairies and others have seen considerable declines in crop productivity due to salinity in lands. The consequence represents a significant economic and political factor and contributes to food shortages in many undeveloped countries.
Although there is engineering technology available to overcome this problem, though drainage and supply of high quality water, these measures are extremely costly. In most of the cases, due to the increased need for extensive agriculture, neither improved irrigation efficiency nor the installation of drainage systems is applicable. Moreover, in the arid and semi-arid regions of the world water evaporation exceeds precipitation. These soils are inherently high in salt and require vast amounts of irrigation to become productive. Since irrigation water contains dissolved salts and minerals, an application of water is also an application of salt that compounds the salinity problem.
Conventional breeding strategies for salt tolerance have been attempted for a long time. These breeding practices have been based mainly on the following strategies: a) the use of wide crosses between crop plants and their more salt-tolerant wild relatives (Rush, P W and Epstein, E, 1981 J. Amer. Soc. Hort. Sci. 106, 699–704) (b) screening and selecting for variation within a particular phenotype (Norlyn, J D. 1980. In: Genetic Engineering of Osmoregulation (Eds. D W Rains, R C Valentine and A Hollaender) pp. 293–309. Plenum press: New York.), c) designing new phenotypes through recurrent selection (Tal, M. 1985. Plant and Soil 89, 199–226). The lack of success in generating tolerant varieties would suggest that conventional breeding practices are not enough and that in order to succeed a breeding program should include the engineering of transgenic crops which allows one to generate salt stress-tolerant crops (Flowers T J and Yeo, A R, 1995. Aust. J. Plant. Physiol. 22., 875–884., Bohnert H J and Jensen, R G. 1996. Aust. J. Plant. Physiol. 23., 661–667.)
Plant cells are structurally well suited to the compartmentation of ions. Large membrane-bound vacuoles are the site for a considerable amount of sequestration of ions and other osmotically active substances. Transport mechanisms could actively move ions into the vacuole, removing the potentially harmful ions from cytosols. Thus, at the cellular level both specific transport systems for sodium accumulation in the vacuole and sodium extrusion out of the cell are correlated with salt tolerance.
Several sodium transport systems associated with salt tolerance have been characterized in different organisms and a few of the genes involved in this process have been identified and in some cases the predicted role of protein has been investigated in transgenic/overexpression experiments. A single gene (sod2) coding for a Na+/H+ antiport has been shown to confer sodium tolerance in fission yeast (Jia, Z P, et al. 1992 EMBO J. 11, 1631–1640., Young, P G and Zheng, P J. Patent#WO9106651), although the role of this plasma membrane-bound protein appears to be only limited to yeast One of the main disadvantages of using this gene for transformation of plants is associated with the typical problems encountered in heterologous gene expression. Two homologues of sodium antiporter, AtNhx1 and SOS1 from salt-sensitive plants, Arabidopsis thaliana have been identified and characterized (Apse, M P et al., 1999, Science 285, 1256–1258., Shi, H et al., 2000 Proc. Natl. Aca. Sci. USA 97, 6896–6901). Overexpression of AtNhx1 in Arabidopsis as well as in fusion yeast shows increased salt tolerance due to better performance of salt compartmentation into the vacuole (Gaxiola, R A., et al. 1999. Proc. Natl. Acad. Sci. USA. 96, 1480–1485., Apse, MP et al., 1999, Science 285, 1256–1258.,). However, a comparison of ion distribution in cells and tissues of various plant species indicates that a primary characteristic of salt-tolerant plants is their ability to exclude sodium out of the cell and to take up sodium and sequester it in the cell vacuoles (Niu, X., et al., 1995 Plant Physiol. 109, 735–742). This strongly suggests that Na+/H+ antiporter from salt-tolerant plants have functionally more effective sodium transport systems compared with salt-sensitive plants such as Arabidopsis.
Therefore, elucidating the function of sodium transport genes in salt tolerant plants will not only advance our understanding of plant adaptation and tolerance to salinity stress, but also may provide important information for designing new strategies for crop improvement Newly generated salt tolerant plants will have many advantages, such as increasing the range that crop plants can be cultivated in salinity lands. This invention fulfills this need by providing the sequences of plant Na+/H+ antiporter genes that are expressed in the halophyte Physcomitrella patens, which can therefore provide a basis of increasing the salt tolerance of non-halophytic plants.
Moreover, the present invention provides novel polynucleotides encoding plant Na+/H+ antiporter polypeptides, fragments and homologs thereof. Also provided are vectors, host cells, antibodies, and recombinant methods for producing said polypeptides. The invention further relates to methods of applying these novel plant polypeptides to the identification, prevention, and/or conferment of resistence to various plant diseases and/or disorders, particularly environmental stress tolerance in plants.
Due to the commercial consequences of environmental damage to crops, there is an interest in understanding how to improve a plant's response to environmental damage. By improving a plant's performance or survival in response to cold, drought and salinity the environment stress-related risks of farming can be reduced. This invention fulfills in part the need to identify new Na+/H+ antiporters capable of conferring drought, freezing and salt tolerance to plants upon over-expression. Namely, we describe Na+/H +antiporters (PpNHX1 and PpNHX2) from Physcomitrella patens. 